In the past few years, low emissivity windows, which allow short wave energy to be transmitted through glass but reflect long wave (infrared) energy, have become very popular on new buildings. These windows are popular because they can save a great deal of money on heating and cooling costs. By reflecting the long wave infrared energy on the outside of the window in the summer radiant heat can not get into the building, therefore the window helps to keep cooling costs down. In the winter, by reflecting the infrared energy on the inside of the window, heat can not escape as easily, therefore the window helps to keep heating costs down.
These low emissivity windows are very efficient for keeping out long wave infrared radiation but transmit a high percentage of visible light and solar short wave infrared radiation which also heat up a room in the summer increasing cooling costs.
In the past highly reflective windows were used to keep the visible and short wave infrared radiation out.
Unfortunately reflecting visible light and short wave infrared radiation on the outside of the window causes a radiation pollution of the immediate environment. In cities the building across from or next to the building with these highly reflective windows gets a lot of unwanted radiation heating the building. This is especially pronounced in modern all glass exterior buildings. This visible light and short wave infrared radiation pollution is also responsible for radiation burning and thus killing vegetation near the building. Because of these problems some cities are forbidding the use of this type of glass despite the advantages of lowered energy use. There is a need for a non reflecting low emissivity glass that will not pollute the environment with reflected visible light radiation and yet keep a substantial portion of the visible solar light and short wave heat from entering and heating a room.